[DataFrames] Updating Error messages to encourage contribution. (#1623)

python/ray/dataframe/__init__.py

@@ -3,7 +3,7 @@ from __future__ import division
 from __future__ import print_function
 import threading
 
-DEFAULT_NPARTITIONS = 10
+DEFAULT_NPARTITIONS = 4
 
 
 def set_npartition_default(n):

python/ray/dataframe/test/test_dataframe.py

@@ -7,6 +7,8 @@ import numpy as np
 import pandas as pd
 import ray.dataframe as rdf
 
+ray.init()
+
 
 @pytest.fixture
 def ray_df_equals_pandas(ray_df, pandas_df):